Eric asked . 2022-03-03

What does the output of imregcorr mean?

%%Start with a clean workspace
clear all;close all;clc;%#ok

%%Load image
fixedFull  = double(imread('cameraman.tif'));
rows = 30:226;
cols = rows;
fixed = fixedFull(rows,cols);

%%Specify motion parameters
theta = 5;%degrees
rowshift = 1.65;%pixels
colshift = 5.32;%pixels

%%Create rotated/translated image
RT = @(img,colshift,rowshift,theta) imrotate( imtransform(img, maketform('affine', [1 0 0; 0 1 0; colshift rowshift 1]), 'bilinear', 'XData', [1 size(img,2)], 'YData', [1 size(img,1)], 'FillValues', 0),theta,'crop'); %#ok
movingFull = RT(fixedFull, colshift, rowshift, theta);
moving = movingFull(rows,cols);

%%Show both images
figure;
imshowpair(moving,fixed,'montage');

%%Register images
[rowshift1, colshift1, theta1, imgReg] = RegisterViaReddy(fixed, moving);
tform1 = imregcorr(moving, fixed, 'rigid');

The function handle RT first translates an image and then rotates it. The resulting image is the same size as the input image. The outputs of my own RegisterViaReddy function are

>> [rowshift1, colshift1, theta1]

ans =

     -1.7600   -5.1000  -5.3402

These are nearly the opposites of the known rowshift, colshift, and theta parameters. I wrote my code this way so that

RT(moving,colshift1,rowshift1,theta1);

generates something that looks like the fixed image.

I do not understand how to get these parameters from the output of imregcorr (tform1). I understand that acosd(tform1.T(1,1)) is 5.1799 and is hence the rotation angle. However, tform1.T is

    0.9959    0.0903         0
   -0.0903    0.9959         0
    4.1423  -10.3337    1.0000

How do I extract meaningful translation parameters from this? I know I can generate something that looks like the fixed image using

imwarp(moving, tform1);

but the resulting array is 214x214 whereas fixed and moving are 197x197. Is there any way to get the translation offsets that I input from the output of imregcorr?

imregcorr , geometric matrix t...